Electrical connector portion for automatic railway couplers

ABSTRACT

Electrical connector portion of an automatic railway coupler including a multiple-contact unit arranged in a support casing carried on one mating head of the coupler for engaging and making electrical connection with a counter-part electrical connector portion carried in similar manner on a counterpart mating head of the coupler, said connector portion being characterized in that said multiple-contact unit is axially slidably mounted in said support casing and in that resilient means are interposed between the multiple-contact unit and the support casing for absorbing impact forces during coupling operations.

United States Patent Prada Oct. 22, 1974 ELECTRICAL CONNECTOR PORTION FOR AUTOMATIC RAILWAY COUPLERS Inventor: Cesare Prada, Turin, Italy Assignee: Compagnia Italiana Westinghouse Freni e Segnali, Turin, Italy Mar. 29, 1973 Filed:

App]. No.:

Foreign Application Priority Data Mar. 3l, 1972 Italy 68029/72 US. Cl. 213/].3, 213/76 Int. Cl. 361g 5/06 Field of Search 213/].3, 76; 339/48 References Cited UNITED STATES PATENTS Jeffrey et al. 213/13 Gnavi 2l3/l.3

h in,

3,646,498 2/1972 Reed 2l3/l.3 X

Primary Examiner-Robert S. Ward, Jr. Assistant Examiner-Gene A. Church Attorney. Agent, or Firm-R. W. Mclntirc, Jr.

[57] ABSTRACT Electrical connector portion of an automatic railway coupler including a multiple-contact unit arranged in a support casing carried on one mating head of the coupler for engaging and making electrical connection with a counter-part electrical connector portion carried in similar manner on a counterpart mating head of the coupler, said connector portion being characterized in that said multiple-contact unit is axially slidably mounted in said support casing and in that resilient means are interposed between the multiplecontact unit and the support casing for absorbing impact forces during coupling operations.

12 Claims, 6 Drawing Figures ELECTRICAL CONNECTOR PORTION FOR AUTOMATIC RAILWAY COUPLERS BACKGROUND OF THE INVENTION Presently known automatic railway car couplers generally are provided with portions for making fluid pressure or pneumatic connections as well as electrical connections carried within a common housing or support means. In the type of coupler presently being discussed, the electrical portion normally comprises a plurality of electrical contact elements for the several train circuits to be connected, each of said contact elements being individually mounted in the support housing and each comprising a plurality of parts with respective resilient means disposed therebetween for cushioning or absorbing impact forces during coupling operation.

The structure immediately above discussed, particularly the arrangement of having the contact elements individually resiliently mounted, is found to be unsatisfactory in that such an arrangement does not provide adequate protection against damage for the contact elements during a coupling action characterized by excessive impact forces between the coupler heads. A further objection to the presently known arrangement of the type of electrical connector above discussed is the difficulty encountered in repairing or replacing damaged parts, which now requires that the entire electrical connector portion be dismounted from the coupler head in order to obtain access to the inner parts of said electrical connector.

SUMMARY OF THE INVENTION The object of the present invention, therefore, is to provide an electrical connector portion or device for an automatic coupling head for railway vehicles which provides protection against damage of the electrical contact elements resulting from excessive impact forces during coupling operation and which permits replacement or repair of damaged parts without dismounting the electrical connector from the coupler head.

The electrical contact portion, according to the invention, comprises a multiple electrical contact unit provided with a plurality of electrical contact elements arranged to make electrical connections with corresponding electrical contact elements, respectively, of another electrical connector portion when the latter is thrust against the former, said connector portion being disposed in a support housing fixed to the coupler head. The electrical contact portion is characterized in that the multiple contact unit thereof is axially slidable, as a unit, on a seat or slide surface formed in the support housing and has resilient means interposed between the contact unit and the support housing for absorbing impact forces during coupling operation.

For a better understanding of the present invention, a description is given hereinafter of a preferred embodiment, by way of example, with reference to the accompanying drawings in which:

FIG. I is a partially sectional perspective view of the electrical connector device according to the present invention;

FIG. 2 is a vertical section through the device shown in FIG. 1;

FIGS. 3 to 6 are fragmentary profile views of a groove and slot formed in a certain part of said device.

showing successive stages of the engagement into said groove and slot of a tooth provided on another part of the same device.

DESCRIPTION AND OPERATION With reference to FIGS. 1 and 2, a portion of a mating head of an automatic railway vehicle coupler is shown on which an electrical connector portion or device embodying the invention is mounted for automatically making electrical connections between railway vehicles, said electrical connector device being indicated overall by the reference numeral 1. This carcoupling portion may conveniently be of any wellknown automatic type and, therefore, a detailed drawing and description thereof is not deemed essential to an understanding of the present invention.

The electrical connector portion I comprises a body 2 (FIG. 2) provided with a base disc 3, a semicylindrical part 4, and a half bush 5 axially slidable with respect to said body and defining with it a single cylindrical element. The half bush 5 is normally thrust towards the left, as viewed in FIG. 2, by an arrangement of helical springs 6 compressed between the base 3 and the opposite inner surface of said half bush. In contact with the base 3 of the body 2 there is an end piece 7 having a cylindrical lateral wall 8 and an end wall 9 provided with a plurality of cavities 10 inside which are seated correspondingly arranged annular projections 11 of male electrical contact elements 12 and female electrical contact elements 13, respectively. The contact elements of the female type are housed in bores I4 in the body 2, whereas those of the male type traverse corresponding bores 15 formed in the half bush 5, which is arranged so as to move axially relatively to the contact elements 12 and 13 from the position shown in a right-hand direction as viewed in FIG. 2, so as to uncover the left-hand or outer ends of the contact elements.

Both male and female contact elements 12 and 13 are provided with respective connection parts 16 for connecting to corresponding electrical conductors 17 disposed within a sheath 18 of substantially tubular form.

The described parts of the electrical connector I are housed within a cylindrical housing 19 (FIG. 2) a front edge 20 of which is bent over so as to form a stop shoulder for a complementarily disposed shoulder 21 of the body 2 and for corresponding projecting parts 22 of the half bush 5.

Further supporting structure for the electrical connector device, according to the invention, comprises substantially a support casing 23 (FIGS. 1 and 2) provided with a front wall 24 and back wall 25, and a pair of lateral walls 26 disposed axially between said front and rear walls and substantially orthogonal thereto and in facing relation to each other. Casing 19, of which FIGS. I and 2 show only the top part, in addition to the electrical connector device I and according to the invention, can also house elements for making pneumatic or fluid pressure connections which, as known, generally form part of automatic coupling heads. The casing 23 is supported by a plate 27 provided with perpendicularly extending arms 28 (only one of which is visible), in FIGS. I and 2, said arms being provided with bores in which are disposed resilient elements 31 respectively. Each of the resilient elements 31 comprises substantially a rubber ring 32 (FIG. 2) disposed between a pair of coaxially arranged metal rings, an inner one 33 and an outer one 34, the inner ring being coaxially traversed by a pin 35 screwed into the casing 23, and the outer ring being fixedly disposed in the bore in the arm 28.

The support casing 23 is provided with a bore 38 (FIG. 2), which traverses the relative walls 24 and 25 and which forms a sliding seat for the electrical connector I. At the front, that is, at the left-hand end as viewed in the drawings, of casing 23, there is a shoulder 40 arranged for cooperating with the bent edge of the housing 19.

Inside the housing 19 there is an annular element or spacer 41 (FIGS. 1 and 2), the left-hand end of which rests against the adjacent annular end of cylindrical wall 8 (FIG. 2) of the end piece 7, while the other end is in contact with an annular resilient gland 42 situated between the end piece and a washer 43, all of which parts coaxially encircle the tubular sheath 18. The washer 43 comprises an axially extending annular projection 44 which coaxially surrounds the gland 42. A bush 45 comprises a portion 46 of such diameter as to fit within the bore of housing 19, and a portion 47 of larger diameter substantially coincident with that of the bore 38 in the casing 23. Portion 46 of bush 45 may be fixed, in a manner to be explained, to the housing 19 by a plurality of radial pins 48 (three in the case of the embodiment shown) each of which comprises a head 51. The gland 42 and washer 43 are housed, as shown in FIG. 2, inside portion 46 of the bush 45.

The annular element 41 is provided with a pair of frontal teeth 52 of different dimensions and arranged to couple with corresponding slots or recesses formed in the adjacent end of cylindrical wall 8 of the end piece 7. Two other frontal teeth 53 are formed in the bush 45 and are arranged for coupling with corresponding slots in the adjacent end of annular element 41. Radially from the external surface of the bush 45 project two diametrically opposite teeth or radial projections 54 (FIGS. 1 and 2), each of which is arranged for insertion into a corresponding axial groove 55 formed in the casing 23. A part of one of said grooves 55 is visible in the perspective sectional representation of FIG. I.

An internal bore in the part 46 of the bush 45 is threaded and has screwed thereinto a threaded end portion of a tubular element 56 slidingly surrounding tubular sheath 18. A further bush 57 is coaxially slidingly supported on tubular sheath l8 and cooperates therewith to form therebetween a coaxially extending annular cavity in which a helical spring 58 is disposed with one end resting against a shoulder comprising the threaded end ofelement 56. Furthermore, the bush 57 at its inner end is provided with a smaller-diameter portion 59 which slides telescopically into portion 47 of the bush 45. The bush 57 is finally provided with an internal annular radial projection or shoulder 60 against which the other end of helical spring 58 rests for causing said shoulder to be thrust against a resilient ring 61 fixed externally to the tubular element 56. A sleeve 62 fixed to and in axial alignment with the bush 57 at the outer end thereof serves as a guide for the sheath 18.

Two diametrically oppositely disposed teeth 64 (FIGS. I and 2) project radially from the external surface of bush 57 and are arranged to cooperate, in a manner to be hereinafter explained, with the axial grooves 55 and with generally arcuate slots 65 (FIGS. 1 and 2) formed in casing 23, each of which slots communicates with a corresponding groove 55 and has its own arcuate axis substantially orthogonal to the axis of the communicating groove. As is clearly visible in FIG. 3, which shows the plan view of the development of the profile of one of the slots 65, each of said slots comprises a first section 66 of substantially uniform width, and a second section 67 ofa greater width than the we ceding width. The two sections 66 and 67 of each slot 65 are connected by a pair of off-set surfaces 71 and 72, the first of which is substantially orthogonal to the lateral surfaces which laterally bound the sections 66 and 67, whereas the second lies at a certain angle relative to said lateral surfaces. Furthermore, in the development shown in FIG. 3, the surface 71 lies in a plane which intersects the surface 72.

The electrical connector device 1 may be conveniently covered by a cap (not shown) rigidly connected with a lug 74 rotatable in a known manner about a vertical axis orthogonal to the axis of the connector.

The assembly and operation of the electrical connector device 1 described is effected in the manner hereinafter set forth.

All the parts of the electrical connector device 1 are mounted inside the cylindrical housing 19, particularly the body 2 (FIG. 2), the half bush 5 and end piece 7, with which are associated the other parts previously described making up the connector devicev Assembly is made while the housing 19 is separated from the casing 23. Afterwards, the annular element 41 is placed inside the housing 19 so that the frontal teeth 52 provided on it are inserted into the corresponding slots in the cylindrical wall 8 of the end piece 7. The bush 45 is inserted by fitting portion 46 inside the adjacent end of housing 19. A preliminary fixing between bush 45 and housing 19 is obtained by inserting the pins 48 radially into them. The gland 42 and washer 43 are assembled with the annular projection 44 of the latter being provided with slots which couple with the heads 51 of pins 48. At this point the tubular element 56 is screwed in, and exerts an axial force on the washer 43 towards the left, as viewed in FIG. 2.

Under the action of the axial force exerted by tubular element 56, the washer 43, gland 42, annular element 41, and piece 7 and body 2 are thrust against the bent edge 20 of the housing 19, and simultaneously a reaction is set up on the bush 45 which tends to move the bush towards the right relative to housing 19, as viewed in FIG. 2. Because of the opposing force acting on gland 42, said gland is radially deformed so as to exert high radial pressure on the sheath 18 of the conductor 17, said radial pressure thus acting to eliminate any play existing between the pins 48 and their respective bores in bush 45 and housing 19, so forming a single rigid assembly.

The assembly so obtained is now inserted inside the bore 38 (FIG. 2) of casing 23, introducing it by its rear end and positioning it so that the teeth 54 of the bush 45 (FIGS. 1 and 2) are inserted into the corresponding axial grooves 55 of the casing 23. In this manner the connector 1 is correctly positioned with respect to the casing 23, such positioning being obtained by the two pairs of teeth 52 and 53 which position the end piece 7 angularly with respect to the annular element 41 and the latter angularly with respect to the bush 4S, and by the teeth 54 which angularly position the bush itself with respect to the casing 23.

The spring 58 and bush 57 can now be assembled on the tubular element 56, which is then inserted rear end first into the casing 23. When inserting this assembly into casing 23, the teeth 64 of bush 57 are guided into the axial grooves 55 in said casing 23, then axially moving the bush towards the left, as viewed in FIG. 2, until the teeth 64 are brought into a position corresponding with the slots 65, then finally rotating the bush clockwise with reference to FIG. 1 so as to push the teeth 64 inside the slots.

Some of the aforementioned stages in assembling the bush 57 are shown diagrammatically in FIGS. 3 to 5. FIG. 3 diagrammatically shows the axial movement of the bush 57 with respect to the casing 23 during which the teeth 64 move into the axial grooves 55. FIG. 4 diagrammatically shows the rotation of bush 57 during which the teeth 64 travel along the first section 66 of the slots 65. FIG. 5 shows the final assembly position for bush 57 in which the teeth 64 are brought up against lateral surfaces 75 of the second section of said slots by the action of the spring 58.

It is evident that by assembling the bush 57 in the manner described, the electrical connector device 1 is resiliently fixed to the casing 23. In fact, the parts rigid with the tubular element 56 and in particular the housing l9 are thrust by the spring 58 against the shoulder 40 of the casing 23, and simultaneously the teeth 64 of the bush 57 are thrust against the lateral surfaces 75 (FIG. 5) of the slots 65.

The device cannot be disassembled accidentally because when the teeth 64 are in the position shown in FIG. 5, corresponding to normal working conditions for the device according to the invention, the surfaces 71 (FIG. 5) of the slot 65 block the teeth 64, so preventing rotation of the bush 57. If under exceptional working conditions external forces are applied to the bush, for example inertia forces, which tend to move it towards the left, as viewed in FIG. 2, a corner of each tooth 64 rests on the surface 72 ofa respective slot 65, as shown diagrammatically in FIG. 6. Consequently, because of the slope of the surfaces 72, forces are applied to the teeth 64 which tend to move them towards peripheral surfaces 76 (FIG. 6) of the slots 65, so as to generate on the bush 57 a moment in the opposite direction to that necessary for dismantling the bush. It is evident that the bush can only be disassembled by first moving it axially towards the left as viewed in FIG. 2, by a predetermined amount equal at least to the dimension d shown in FIG. 6. and then after said movement, applying to the bush a moment in such a direction as to move the teeth 64 into the sections 66 of the slots 65.

To use the electrical connector device 1, which is possible after the protective caps (not shown) have been opened by rotating the lug 74 (FIG. 1), an axial force is applied to the half bush 5 (FIG. 2) by the part 4 of the body 2 of a corresponding electrical connector device arranged on a counterpart mating head for coupling to the first electrical connector device, moves the half bush 5 towards the right, as viewed in FIG. 2, and makes the connection between the male and female contact elements 12 and 13. respectively, of said connector devices. This force, which is transmitted to the body 2 by way of the spring 6 and annular element 56, is opposed by the helical spring 58 which consequently provides a resilient support during said connection operation. The rigidity of spring 58 must be chosen so as to enable the described operation to be carried out smoothly and regularly.

If by accident an axial force is applied to the body 2 of a much greater value than that necessary for carrying out the described connection operation, this force deforms the spring 58 and moves the housing 19 together with all the parts associated therewith axially towards the right, as viewed in FIG. 2, both with respect to the casing 23 and to the bush 57, so setting up a telescopic movement between the latter and the bush 45. The travel permitted during such relative movement may be conveniently equal to that necessary for bringing a front face 77 of the body 2 into substantial alignment with a front face 78 of the casing 23. This avoids damaging the body 2 and the other parts of the connector device 1.

It is evident that modifications and variations, both of form and disposition of the various parts, may be made to the embodiment described in the present invention without leaving the scope of the invention itself.

Having now described the invention, what I claim as new and desire to secure by Letters Patent, is:

1. An electrical connector device for an automatic coupler mating head for railway vehicles, said connector device comprising a cylindrical support casing fixedly mounted on the mating head and having a longitudinal groove formed internally and parallel to the axis thereof, a multiple contact unit including a cylindrical housing coaxially slidably mounted in said support casing and connected to an axially aligned bush having a radial projection formed thereon for riding in said groove for accommodating sliding movement of said contact unit within said support casing, said contact unit having a plurality of electrical contact elements an ranged thereon to make respective electrical connections with correspondingly arranged electrical contact elements of a counterpart electrical connector device carried on a counterpart mating head when the latter head is thrust against the former, and a spring interposed between said support casing and said contact unit for resiliently cushioning thrust impact between the two contact units during coupling action of the mating heads.

2. An electrical connector device, as claimed in claim 1, characterized in that said support casing is provided at one end thereof with stop means for limiting axial sliding movement, in one direction, of said contact unit to a predetermined position in said slide groove when the coupler mating heads are in an uncoupled relation, in which predetermined position the external extremity of said contact unit projects outwardly from and beyond the corresponding extremity of the support casing by a predetermined amount relative to each other.

3. An electrical connector device, as claimed in claim 2, characterized in that said spring is of such a compression rating as to provide a resisting force of a certain value in opposition to axial sliding movement of said contact unit in a direction opposite to said one direction when said electrical connector devices are thrust against each other during coupling operation in order to make said electrical connections, said spring being effective for absorbing a part of the opposing forces which the contact units transmit to each other during said coupling operation and being yieldable to a coupling force acting oppositely to and exceeding said resisting force for effecting retraction of the contact unit within said support casing until the respective corresponding extremities of said contact unit assume relative coplanar positions.

4. An electrical connector device, as claimed in claim 1, characterized in that the connection between said housing and said bush is made by a plurality of radially disposed, angularly spaced pins inserted therebetween.

5. An electrical connector device, as claimed in claim 4, in which said electrical contact elements of said contact unit are connected by corresponding electrical conductors disposed in a tubular sheath, characterized in that within said bush there is coaxially disposed an annular resilient gland coaxially surrounding and supporting said sheath, said gland being coaxially interposed between and making abutting contact on opposite sides with an annular spacer in contact with said contact unit, and one end of said bush, said bush and said spacer being provided with respective teeth for engaging respective recesses formed in said spacer and the contact unit for angularly positioning the several parts relative to each other and within the connector device.

6. An electrical connector device, as claimed in claim 5, characterized in that said bush is provided with an internally threaded bore into which a screw-threaded end of a tubular element coaxially surrounding said sheath is screwed and arranged for exerting an axial force on said gland in a direction corresponding to said one direction and acting through the front end of said housing, said gland and said spacer for biasing said contact unit toward said stop means and said predetermined position.

7. An electrical connector device, as claimed in claim 6, in which said spring is a helical spring coaxially externally encircling said tubular element and is compressed, said spring is disposed between the screwthreaded end thereof and a radial shoulder of a second bush coaxially slidably disposed within said support casing at the end opposite said contact unit.

8. An electrical connector device, as claimed in claim 7, characterized in that said second bush is also provided with at least one radial projection arranged for insertion into said axial slide groove in the support casing, with each of said axial grooves there being associated a second groove formed in said support casing perpendicularly to and connecting with the axial groove and being arranged to receive said radial projection formed on said second bush in such a manner as to constitute an axial stop for said second bush.

9. An electrical connector device, as claimed in claim 8, characterized in that said second groove comprises a first straight section of a certain width substantially orthogonal to the axis of and connecting with the axial groove and a second section of greater width than and connecting with the first section, said second section constituting said axial stop for said radial projection of the second bush.

10. An electrical connector device, as claimed in claim 9, characterized in that each pair of surfaces which laterally bound said first and second sections of said second groove form respective offset surfaces arranged for locking said second bush against rotation when said radial projection of said second bush is disposed within said second section of said second groove.

11. An electrical connector device, as claimed in claim 10, characterized in that one of said offset surfaces is substantially orthogonal to the axis of said second groove, and the other offset surface is disposed at an angle less than relative to said axis of said second groove.

12. An electrical connector device, as claimed in claim ll, characterized in that said one of said offset surfaces is formed on that side of said second groove against which said radial projection of the second bush is biased by rests under the axial force exerted by said spring acting in a direction corresponding to said direction opposite to said one direction. 

1. An electrical connector device for an automatic coupler mating head for railway vehicles, said connector device comprising a cylindrical support casing fixedly mounted on the mating head and having a longitudinal groove formed internally and parallel to the axis thereof, a multiple contact unit including a cylindrical housing coaxially slidably mounted in said support casing and connected to an axially aligned bush having a radial projection formed thereon for riding in said groove for accommodating sliding movement of said contact unit within said support casing, said contact unit having a plurality of electrical contact elements arranged thereon to make respective electrical connections with coRrespondingly arranged electrical contact elements of a counterpart electrical connector device carried on a counterpart mating head when the latter head is thrust against the former, and a spring interposed between said support casing and said contact unit for resiliently cushioning thrust impact between the two contact units during coupling action of the mating heads.
 2. An electrical connector device, as claimed in claim 1, characterized in that said support casing is provided at one end thereof with stop means for limiting axial sliding movement, in one direction, of said contact unit to a predetermined position in said slide groove when the coupler mating heads are in an uncoupled relation, in which predetermined position the external extremity of said contact unit projects outwardly from and beyond the corresponding extremity of the support casing by a predetermined amount relative to each other.
 3. An electrical connector device, as claimed in claim 2, characterized in that said spring is of such a compression rating as to provide a resisting force of a certain value in opposition to axial sliding movement of said contact unit in a direction opposite to said one direction when said electrical connector devices are thrust against each other during coupling operation in order to make said electrical connections, said spring being effective for absorbing a part of the opposing forces which the contact units transmit to each other during said coupling operation and being yieldable to a coupling force acting oppositely to and exceeding said resisting force for effecting retraction of the contact unit within said support casing until the respective corresponding extremities of said contact unit assume relative coplanar positions.
 4. An electrical connector device, as claimed in claim 1, characterized in that the connection between said housing and said bush is made by a plurality of radially disposed, angularly spaced pins inserted therebetween.
 5. An electrical connector device, as claimed in claim 4, in which said electrical contact elements of said contact unit are connected by corresponding electrical conductors disposed in a tubular sheath, characterized in that within said bush there is coaxially disposed an annular resilient gland coaxially surrounding and supporting said sheath, said gland being coaxially interposed between and making abutting contact on opposite sides with an annular spacer in contact with said contact unit, and one end of said bush, said bush and said spacer being provided with respective teeth for engaging respective recesses formed in said spacer and the contact unit for angularly positioning the several parts relative to each other and within the connector device.
 6. An electrical connector device, as claimed in claim 5, characterized in that said bush is provided with an internally threaded bore into which a screw-threaded end of a tubular element coaxially surrounding said sheath is screwed and arranged for exerting an axial force on said gland in a direction corresponding to said one direction and acting through the front end of said housing, said gland and said spacer for biasing said contact unit toward said stop means and said predetermined position.
 7. An electrical connector device, as claimed in claim 6, in which said spring is a helical spring coaxially externally encircling said tubular element and is compressed, said spring is disposed between the screw-threaded end thereof and a radial shoulder of a second bush coaxially slidably disposed within said support casing at the end opposite said contact unit.
 8. An electrical connector device, as claimed in claim 7, characterized in that said second bush is also provided with at least one radial projection arranged for insertion into said axial slide groove in the support casing, with each of said axial grooves there being associated a second groove formed in said support casing perpendicularly to and connecting with the axial groove and being arranged to receive saiD radial projection formed on said second bush in such a manner as to constitute an axial stop for said second bush.
 9. An electrical connector device, as claimed in claim 8, characterized in that said second groove comprises a first straight section of a certain width substantially orthogonal to the axis of and connecting with the axial groove and a second section of greater width than and connecting with the first section, said second section constituting said axial stop for said radial projection of the second bush.
 10. An electrical connector device, as claimed in claim 9, characterized in that each pair of surfaces which laterally bound said first and second sections of said second groove form respective offset surfaces arranged for locking said second bush against rotation when said radial projection of said second bush is disposed within said second section of said second groove.
 11. An electrical connector device, as claimed in claim 10, characterized in that one of said offset surfaces is substantially orthogonal to the axis of said second groove, and the other offset surface is disposed at an angle less than 90* relative to said axis of said second groove.
 12. An electrical connector device, as claimed in claim 11, characterized in that said one of said offset surfaces is formed on that side of said second groove against which said radial projection of the second bush is biased by rests under the axial force exerted by said spring acting in a direction corresponding to said direction opposite to said one direction. 